1. Field of the Invention
The invention relates to optical devices; more particularly, it relates to optical circulators.
2. Description of Related Art
An optical circulator is a nonreciprocal, typically three-port or four-port, device. Light entering the first port passes out the second port, but light entering the second port cannot pass back to the first port. Instead, it passes out of the third port. By installing an optical circulator at each end of a fiber link, an existing unidirectional fiber optic communication link can be quickly and economically converted to a bi-directional one. Such a modification results in a doubled bit carrying capacity. An optical circulator can also be used in applications such as wavelength division multiplexer (WDM), Erbium-doped fiber amplifier (EDFA), add-drop multiplexers, dispersion compensators and optical time domain reflectometers (OTDR's).
Optical circulators can be a key element in today's optical networks. However, they have not been widely adopted because of their high cost. A typical optical circulator usually comprises many optical elements and has a large optical footprint. Manufacturing of conventional optical circulators usually requires precise alignment of each optical element, leading to low yields and high production costs.
An early concept of a polarization independent optical circulator for telecommunication use was disclosed in Matsumoto, U.S. Pat. No. 4,272,159. This document, and all others referred to herein, are incorporated by reference as if reproduced fully herein.
Optical circulators have been described in patents, including the above-mentioned Matsumoto, U.S. Pat. No. 4,272,159; Emkey, U.S. Pat. No. 4,464,022; and Kuwahara, U.S. Pat. No. 4,650,289. However, these early optical circulators often suffer from high insertion loss and/or cross-talk that is unacceptable for many communication applications. Insertion loss may be defined as the difference between the power between light launched into the optical circulator and the power that exits the device. Insertion loss is largely due to coupling loss from fiber to fiber, absorption of light and to imperfect polarization separation. Cross-talk in an optical circulator refers to the amount of power emitted at an optical port to the receiver from light entering at an adjacent optical port from the transmitter. The conventional polarizing cubes used in these prior optical circulators often cause large insertion loss and cross-talk because of their low polarization extinction ratio.
Optical Circulators Using Beam Shifters
Koga, U.S. Pat. Nos. 5,204,771; 5,319,483 and Cheng U.S. Pat. Nos. 5,471,340; 5,574,596, disclose optical circulators using beam shifters. The beam path determining means of these patents shift a beam such that it possesses the same propagation direction but is spatially located in a different portion of the circulator. In this sense, the input beam to and output beam from the beam path determining means are parallel in propagation direction but are shifted in spatial location. A disadvantage of the Koga and Chen circulators is that the construction of these circulators demands precise fabrication of birefringent crystals and waveplates. These types of circulators are therefore often difficult and costly to make. The length of beam shifter in these circulators required to obtain adequate beam separation is also excessively large thus resulting in a large form factor.
Another drawback of the Cheng circulators is that polarization mode dispersion ("PMD") in the circulators is not eliminated unless additional compensation crystals are introduced. Such additional crystals add cost and complexity. Polarization mode dispersion (PMD) is introduced in an optical component when signal energy at a given wavelength is resolved into two orthogonal polarization modes of slightly different propagation velocity or optical path. The resulting difference in propagation time between polarization modes is also called differential group delay. PMD causes a number of serious capacity impairments, including pulse broadening. In addition, alignment of this type of circulators depends on sub-micron precision positioning of single mode fibers. Therefore, manufacturing of PMD-corrected Cheng circulators is non-trivial.
FIGS. 1A-B show respectively an isometric and a cross-sectional view of a walk-off crystal such as that employed in the Cheng and Koga references. Walk off crystals can be used either for splitting a natural light beam into orthogonally polarized rays, or for circulating light beams with orthogonal polarization components. FIG. 1A shows the later case in which a light beams 150-152 with orthogonal polarization states, circulate between respectively ports 106-104 and ports 102-106 of walk-off crystal 100.
FIG. 1B is a cross-sectional view at principal plane ABCD of the crystal 100 shown in FIG. 1A. The optical axis 108 of the crystal is located in the principal plane and at an acute angle that is typically at around 45 degree with respect to the front surface of birefringent crystal, defined by the plane including AD. The polarization vector, i.e. electric field vector, 118 of ray 150 is normal to the principal section. Thus the propagation vector 124 and Poynting vector 126 for the ray 150 are substantially collinear and no walk-off is exhibited as the ray passes through the crystal to port 104. The polarization, i.e. electric field vector, 116 of ray 152 is parallel to the principal section. Thus the propagation vector 120 and Poynting vector 122 for the ray 152 are not collinear and walk-off is exhibited as the ray passes through the crystal to port 106. The complete explanation of this walk-off effect can be found using electromagnetic theory as embodied in Maxwell's equations. Further explanation, using Huygen's principle, may be found in Hecht, Optics 288 (1987) (2d ed. Addison-Wesley).
Optical Circulators Using Beam Benders
Pan et al., U.S. Pat. Nos. 5,682,446; 5,818,981; 5,689,367 and 5,689,593, describe another type of circulator in which optical ports, beam splitters and non-reciprocal rotators are radially arranged about a polarization sensitive prism pair and associated air gap. Circulation is achieved by polarization sensitive reflection or transmission of an incident light beam from or through the air gap defined between the prism pair as shown in FIG. 2. The length of the beam splitters coupled with the radial arrangement of the ports makes for a circular form factor. The arrangement is bulky and expensive.
FIG. 2 shows an isometric view of a circulating element such as that employed in the Pan et al. references. Prism pair 208, 212 defines an air gap 210 between internal faces 208A, 212A. The prism pair and air gap function as an optical circulator 200 by reflecting and transmitting orthogonally polarized light beams respectively 250-252. Light beam 250 with polarization vector 222 propagates between ports 204 and 202 by entering prism 208 at a normal to face 208C, by internally reflecting off face 208A and air gap 210 within prism 208 and by exiting the prism 208 on a normal to face 208B toward port 202. Light beam 252 with polarization vector 220 propagates between ports 206 and 202 by entering prism 212 on a normal to face 212B, by transmission through gap 210 into prism 208 on a normal to face 208A and by exiting the prism 208 on a normal to 208B toward port 202.
In order to achieve this effect, i.e. polarization sensitive transmission and reflection, several requirements must be met. First, the prisms must have an optical axis. Second, the prisms 208, 212 are separated by an air gap 210 defined between opposing interior faces 208A, 212A of respectively prisms 208, 212. The gap must be greater than the wavelength of the light being transmitted and the interior faces should be parallel. Third, ray 250 must intercept the gap at an angle of incidence greater than a critical angle .theta.c where .theta.c=Arc Tan(n) and n is the corresponding index of refraction of prism 208 for the polarization vector 222 of ray 250. Fourth, each port enters the corresponding one of the prism pairs through a dedicated face at an angle normal to the face. Fifth, the angle 230 between ports 202-204 is a large angle, e.g. 80.degree., since the internal reflection experienced by beam requires that the beam enter the prism at a near grazing angle of incidence with respect to the internal face 208A through face 208C.
Given the above-mentioned problems with prior art optical circulators, there is a need for a simplified optical circulator comprised of simple optical elements with reduced polarization mode dispersion that is suitable for volume manufacturing.